Backwards compatibility for Durable Functions in-process

[cgillum]

Users of today's Durable Functions may want to migrate their code to the .NET Isolated worker. To make this migration process easier, we should consider adding backwards compatibility support. This would ideally be implemented as a set of alternate interfaces that users can bind to whose implementations call into the "modern" APIs. These back-compat interfaces should also be isolated into a separate namespace, like Microsoft.DurableTask.Compatibility.Functions.